Coprinus subpurpureus should be distinguishable from C. plagioporus by its pileocystidia with a cylindrical neck and not or weakly enlarged but not capitate apex and its more purple colour. Collections that have pileocystidia with a subcapitate apex have to be named C. plagioporus because the apex of the pileocystidia in C. plagioporus is not always distinctly capitate.
It should mentioned that there is a great difference between the size of the spores given in the original description and that in our own measurements on the type material (12-14 x 7-8 µm versus 9.0-11.5 x 5.6-7.0 µm).Coprinus fallax is another probably closely related species; but should have a more reddish spore print (see also the discussion under C. plagioporus and C. fallax).